Recent Advances in Designing Molecular Assemblies

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  • Recent Advances in Designing Molecular Assemblies

    The formation of molecular aggregates is an important areaof research that combines a number of chemical expertiseincluding synthesis, spectroscopy, electronic structure theory,and finally device fabrication. Indeed, the study of thephotophysical properties of various molecular aggregates hasreceived a great deal of intention over the last several decades.Both J- and H-aggregates have been examined in great detail,and the electronic structure of these systems has beendocumented and fairly well agreed upon. Recently, there hasbeen renewed interest in the area of photophysical andelectronic properties of molecular aggregates (and modifica-tions) for a number of reasons. One such reason is that with theemergence of new experimental and theoretical methods, it isnow possible to probe the excitations and experimentalparameters in both J- and H-aggregates and their modificationswith greater detail. Also, with better theoretical methods, onecan also compare more closely experimental results to theory.Another reason for renewed interest in this area has been thatthe arrival of new and interesting organic aggregate structuresand assemblies has also opened new avenues for the study ofinter- and intramolecular interactions that might be useful foroptical and electronic applications. These materials are stillfairly new in their characterization status, and many of themethods developed to study other aggregates are now beingapplied to these new and interesting materials. This brings us tothe subject of the current two Perspectives in this issue. Thesetwo Perspectives are written by experts in the field focusing onthe details of the mechanism of excitations in H-aggregates andthe use of the relatively new noncovalent modified graphene forintermolecular interactions with organic chromophores.The first Perspective critically re-examines the photophysics

    of para-distyrylbenzevne (DSB) as an illustration of theherringbone-arranged H-aggregate. While there have been agreat deal of reports regarding this configuration, there is anapparent contradiction in discussing the process of aggrega-tion-induced emission quenching in H-aggregates with theobserved highly emissive DSB crystals. The authors have setout to resolve this particular issue by looking also into theexciton states in single- and polycrystalline systems. It wasfound that DSB forms pronounced H-aggregates, which arehighly luminescent in single crystals but become low emissivein polycrystalline NP suspensions and vapor-deposited or spin-coated films. The authors now provide good reasons for thisbehavior primarily based on low trap concentrations in thesingle crystals, structural defects, and longer lifetimes in theexcited states in the case of polycrystalline samples. There isindeed a strong effect due to the formation of polarons, triplets,and contaminations as well as other structural dislocations thataffect the DSB fluorescence quantum yield. The Perspectivealso clears up the issue regarding how one might increase theeffective emission from low quantum yield polycrystallinesamples with the use of proper sensitization experiments. ThePerspective is a prelude to much more to come in this area andserves as a litmus test for those working in this area to bemindful of making proper assignments, valid structure

    property relationships, and correct interpretations in discerningnew and exciting optical and photophysical properties ofconjugated materials such as DSB.The Pecond perspective involves the properties of single- and

    few-layer graphene. In particular, the contribution looks at therecent interest in interfacing functional molecules and materialsto graphenes high surface area basal plane. Indeed, there is aneed now for good of pure graphene material for this purpose,and there have been many attempts to covalently modify thesurface of graphene. However, the surface of pristine graphenelacks chemical functionality to allow for covalent modificationwithout interrupting its continuous -orbital system. In contrastto this, the authors point out that noncovalent functionalizationdoes not suffer from these limitations. This immediately bringsabout potential applications for such methods and materialsincluding molecular recognition. The authors describe anumber of new strategies to interface molecular compoundsand polymers as well as graphene. This process providesenhanced properties and functions in the prepared materials.For example, in the case of graphene, noncovalent function-alization strategies do not affect the transparency orconductivity of the material as is found in covalentmodification. This in effect offers noncovalent functionalizationas an avenue to prepare other important materials for use inelectronics.The assembling of molecules on the graphene basal plane

    through van der Waals interactions represents an operationallysimple method to introduce arbitrary functionality to thegraphene surface and to influence its charge carrier density.Graphenes atomically precise structure makes it suitable forcrystallizing molecules in two dimensions and has inspired theauthors efforts to control molecular orientation throughmultivalent binding interactions. In the Perspective, the authorspoint out several considerations that are important forevaluating molecular assembly on graphene. Differences inmolecular size and coverage, which can range from fractions ofmonolayers to multilayers, strongly influence the accessibility ofthe graphene basal plane and its electronic properties. It is alsoimportant to match the stability of the adsorbed monolayers ormultilayers to the desired application. According to the authors,graphenes single-atom thickness, desirable electronic proper-ties, and mechanical stability have already attracted interest fortransparent electrodes and FETs and are likely to be leveragedwithin other device platforms. This may also lead to its use insensing and integrated logic circuits. The process of molecularfunctionalization provides ultrathin layers or well-definedmonolayers that have served as dopants and as nucleationsites to ensure conformal oxide coatings through atomic layerdeposition.These two Perspectives provide a nice review and forward-

    looking view of the field of organic aggregates and surfacemodification of important organic assemblies such as graphene.The use of important optical methods to probe the properties

    Published: August 15, 2013


    2013 American Chemical Society 2705 | J. Phys. Chem. Lett. 2013, 4, 27052706

  • of the materials as well as the use of electronic structurecalculations is now to the point that one can readily apply thesemethods to newly prepared materials and provide sufficientrational for the observed behavior. This was certainly the casein the analysis of the DSB H-aggregates as well as the formationof noncovalent modifications of graphene. With furtherinteraction in this field in these two areas, it is expected thatnew and interesting properties may be discovered and put touse in new applications.

    T. Goodson, IIIThe University of Michigan

    AUTHOR INFORMATIONCorresponding Author*E-mail: tgoodson@umich.eduNotesViews expressed in this Editorial are those of the author andnot necessarily the views of the ACS.

    RELATED READINGS(1) Gierschner, J.; Luer, L.; Milian-Medina, B.; Oelkrug, D.; Egelhaaf,H.-J. Highly Emissive H-Aggregates or Aggregation-Induced EmissionQuenching? The Photophysics of All-Trans para-Distyrylbenzene. J.Phys. Chem. Lett. 2013, 4, 26862697.(2) Mann, J. A.; Dichtel, W. Noncovalent Functionalization ofGraphene by Molecular and Polymeric Adsorbates. J. Phys. Chem. Lett.2013, 4, 26492657.

    The Journal of Physical Chemistry Letters Editorial | J. Phys. Chem. Lett. 2013, 4, 270527062706